This invention relates to methods and materials for protecting permeable substances against biodegradation and the like, and more particularly relates to methods and treatment materials for impregnating wood, paper, cordage, and soil and the like with a fungicidal agent.
It is well known that wood is subject to attack by various deleterious agents such as fungi, insects, animals, etc., and it is also well known to impregnate wood and the like with various materials for the purpose of combatting such attacks. The characteristics and nature of these attacks are such that no single chemical or other material has been found to be a universal remedy, and thus it is often desirable to employ more than one type of treatment for wood and the like which is expected to be exposed to more than one type of attack. For example, it is often desired to paint the surface of wood which has also been impregnated with a preservative. Since the effects provided by one type of treatment may render the wood unsuitable for receiving another type of treatment, however, this has not always been practicable. Furthermore, recent changes in social and political attitudes with respect to the human environment have also tended to limit the use of certain treating materials previously deemed to be useful on a broader scale. Also, the cost of other treating materials has also limited their use.
Referring in particular to fungicides as a class, many different materials have been proposed and even used for this purpose. In general, however, approximately 75% of all wood which is treated with a fungicide is now impregnated with creosote, about 5% of such wood is treated with an aqueous solution of one or more salts of copper, chromium and arsenic, and about 20% is impregnated with a polychlorophenol. Creosote is both effective and long-lasting as an effective fungicidal agent. However, the popularity of creosote is attributable more to the fact that it is relatively cheap and simple to handle, since it also has a very strong and unpleasant odor which is difficult if not impossible to mask, and since wood treated with creosote is unpleasant to touch and handle. In addition, wood impregnated with creosote does not effectively receive most paints, and thus creosote-treated wood is undesirable if not unsuitable in most dwellings and the like. Furthermore, creosote is inherently foreign to wood, and thus wood tends to resist impregnation with such material.
Salts of copper, chromium and arsenic, and the like, provide an effective fungicide, as described in U.S. Pat. No. 3,080,212. These salts are quite soluble in water, and since it is far easier to impregnate wood with an aqueous solution, they are much easier to use for wood preservation purposes than creosote. Furthermore, wood treated in this manner is not inhibited against receiving paint. On the other hand, these materials are also quite expensive and, since they are water-soluble, they tend to leach out of wood which is exposed to the elements. Not only does this shorten the life of wood treated with this type of preservative, but these leached-out salts are considered a detrimental factor in the environment.
As hereinbefore stated, the third important class of wood preservatives includes the various polyhalophenols, and more particularly the polychlorophenols such as tetrachlorophenol, pentachlorophenol and, to a limited extent, trichlorophenol. Pentachlorophenol is by far the most widely used of this class of materials. Since all of these polyhalophenols have similar handling and fungicidal characteristics, however, reference will hereinafter be made to "PCP" or to polychlorophenol as denoting any polyhalophenol.
PCP-type fungicides are at least as effective as creosote insofar as their fungicidal properties are concerned, and, although they are not classified as toxic per se with respect to most insects and wildlife, wood treated with PCP tends to be avoided by many harmful insects. In addition, PCP materials are substantially cheaper than creosote. On the other hand, PCP is not water-soluble to any significant degree, and, although this is an advantage insofar as the problem of leaching is concerned, this characteristic required that PCP be employed with an organic solvent (usually No. 2 heating oil or the like). These solvents are themselves relatively expensive, which therefore significantly increases the cost of a PCP treatment operation. In addition, such solvents have the side effect of rendering the wood substantially non-paintable. Furthermore, organic solvents of this type are flammable at temperatures often experienced in wood-treating operations and must therefore be given special handling in order to safeguard personnel and property. Moreover, various environmental protection laws now require that special measures be taken to guard against escape of these solvents into uncontrolled areas, and this has added significantly to the disadvantages of using PCP fungicides rather than creosote.
It will be apparent from the foregoing that many if not most of the disadvantages attendant the use of PCP as a wood preservative may be attributed to the solvent being used, rather than to the PCP itself. As hereinbefore stated, however, PCP is substantially insoluble in water, and even when a non-flammable organic solvent has been used, the result has either been no more advantageous than when heating oil is used, or other more serious limitations have been encountered.
One attempt to avoid the problems of heating oil has involved the use of a liquified petroleum gas, such as propane or butane, as a solvent for treating wood with PCP. More particularly, the PCP is first dissolved in the propane by means of a suitable co-solvent, and the resulting mixture is then injected into the wood by means of conventional pressuring techniques. Thereafter, the propane is evaporated to deposit the PCP in the wood, with the vaporized propane being evacuated from the system.
This technique has at least one advantage over other conventonal treatment methods using PCP in that wood treated in this manner is not inhibited with respect to paintability. However, wood treated in this manner also becomes quite hard, and this renders the treatment less desirable for telephone poles since the wood cannot be penetrated as easily with climbing spikes. Moreover, vaporized propane and the like cannot effectively be reclaimed without the use of special treatment. Not only does this greatly increase costs, but the explosive character of propane in the vapor system creates an operating hazard which is even more dangerous than where a solvent such as heating oil is used.
As an alternative to the foregoing technique, it has also been proposed to use methylene chloride as a solvent for PCP to avoid the explosion hazard which is present with the use of propane and the like. This also has concurrent disadvantages, since methylene chloride is not only quite expensive in comparision with more conventional solvents, but the treatment must be maintained at either a relatively low temperature, or under pressure, or both, to prevent escape through evaporation which will further increase cost.
It has been proposed to impregnate the wood or other material with an aqueous solution of a polychlorophenate salt, and thereafter to inject an acid into the impregnated wood to precipitate the polychlorophenol in situ. A two-stage process is, of course, inherently undesirable since it requires an interim step of drying the wood after the first step to make room for the second treatment solution, and because it also requires more elaborate facilities. In this instance, however, a more serious disadvantage arises from the fact that the reactant must travel from the exterior to the interior of the wood, to achieve full permeation of the wood sought to be treated. PCP is a solid material, except when in an organic solution, and thus the PCP tends to precipitate out of solution and clog the pores of the wood at the point where the acid encounters the Na (PCP) in the wood. Since this occurs in the perimeter zones first, this effectively seals off the interior zones of the wood from the acid, and this, in turn, keeps the wood from becoming fully saturated with PCP precipitate.
These disadvantages of the prior art are overcome with the present invention, and novel methods are provided for impregnating wood with PCP at a greatly reduced cost and in a manner to the hazard of fire or explosion because of the solvent used. More particularly, wood treated by means of the present invention is fully paintable, and is fully impregnated with PCP without adversely affecting its paintability.